1. Field of the Invention
The present invention relates to an illumination device for a microscope.
2. Description of the Related Art
In general, a microscope having a Koehler illumination comprises a field stop function to illuminate the same range as an observation range. For example, when determining a part in a visual field as an observation range at the time of a fluorescence observation, there is adopted a mode that reduces a field stop and illuminates only a necessary part in the visual field in order to avoid a deterioration of a specimen outside the observation range or discoloration of fluorescence.
Further, in recent years, there are FRAP (Fluorescence Recovery After Photobleaching) observation to observe movement of materials in a cell and FLIP (Fluorescence Loss in Photobleaching) observation by partially discoloring in the visual field at the time of fluorescence observation and utilizing its recovery state. Alternatively, there is an observation method to dye with a caged reagent having characteristics chemically sealed therein, restore the characteristics only at an illuminated part by partially applying rays of illumination light and observe its diffusion. These observation methods require means capable of illuminating a part in the visual filed with an arbitrary size or shape at the time of, e.g., fluorescence observation.
In order to illuminate a part in the visual field, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-134250 and PCT National Publication No. 2000-502472 disclose a liquid crystal stop mode that determines a transmission liquid crystal device (LCD) arranged at an aperture position as a field stop. This mode not only match an observation range with an illumination rage by a control over shading of the transmission liquid crystal device but also can illuminate a part in the visual field with an arbitrary size or shape.
Furthermore, PCT National Publication No. 2000-502472 proposes a use of a DMD (Digital Micromirror Device) with a different light modulation structure in place of the liquid crystal device (LCD).
In general, the DMD comprises a protection glass in order to protect a micromirror. Rays of light reflected from a surface of the protection glass and rays of light reflected from gaps existing between micromirrors adjacent to each other, but they are weak, become rays of stray light. Since the rays of stray light do not depend on a control over the micromirrors, they always illuminate an entire specimen including an unnecessary range. Therefore, the specimen cannot be completely protected from the stray light beam by only the control over the micromirrors.
The rays of stray light from the protection glass and the gaps between the micromirrors are much weaker than the rays of light reflected from the micromirrors and do not lead to a problem in many cases. However, when an intensity of rays of illumination light is high or when an illumination time of rays of illumination light is long, affection to the specimen must be taken into consideration.
For example, in general fluorescence observation, even in a case that only one cell in cells cultured in a laboratory dish is to be illuminated, cells other than an observation target are irradiated with rays of illumination light due to the stray light. When the stray light is applied for a long time, cells other than the observation target may possibly have discoloration or may be weakened in some cases.
Moreover, in the above-described FRAP observation, since rays of illumination light having a high intensity is applied to a range in which discoloration is performed, an affect of the stray light cannot be ignored even if it is the stray light having a relatively low intensity. When an application time is long in particular, the entire specimen including a range in which discoloration is not necessary is undesirably subjected to discoloration in no small measure due to the stray light. Therefore, in the FRAP requiring discoloration on a partial fluorescence level, even if regular partial discoloration processing is carried out, there is a possibility that a contrast may be reduced when performing fluorescence observation on the entire specimen. When the contrast is low in this manner, not only the observation cannot be accurately performed, but an observation result cannot be obtained in some cases.